The band were a little over halfway through their 21-song headline set when frontman Dave Grohl announced that they would be joined by one of his “favourite people”, and that she’ll also appear on their new record ‘Concrete & Gold’ when it’s released on September 15.

When Mosshart arrived onstage, dragging on a cigarette, the crowd roared and Grohl joked that she had warned the band to expect an exuberant welcome from the Polish crowd. “She’s the one backstage who said: ‘Wait till you get out there, those people are fucking crazy’,” said Grohl with a grin. “We have a new song called ‘La Di Da’ which we duet on, actually. You wanna hear it?”

After playing the heavy new tune, during which Grohl and Mosshart shared a mic, Grohl thanked her with the words: “Alison Mosshart everyone! From The Kills! Goodness gracious mercy, that was some real rock’n’roll shit!”

Aside from that new song, Foo Fighters’ played a largely similar set to their recent Glastonbury headline performance, once more proving they’ve learned to ‘Walk’ before they ‘Run’.

Prior to the Foo Fighters’ headline set, MIA had played a superbly frenzied set on the festival’s Tent Stage. As well as playing hyperactive versions of fan favourites like ‘Bucky Done Gun’, ‘Bad Girls’ and ‘Paper Planes’, she also used the opportunity to announce that she has deleted her Twitter account. “You’ll just have to come out and see me now,” she said.

She was preceded in the Tent by Jimmy Eat World, who played six tracks from their classic 2001 album ‘Bleed American’, including the title track, ‘A Praise Chorus’, ‘Sweetness’ and closing with perennial indie disco favourite ‘The Middle’. Frontman Jim Adkins announced that it was the band’s “very first time in Poland”, before adding: “Thanks for coming out. Holy shit, there’s a lot of you.”

Earlier in the day, it was also a Polish debut of sorts for Charli XCX, whose amphetamine-rush live show opened the Main Stage. She expressed her excitement at playing her “first ever show in Poland on my own,” having previously visited the country as Katy Perry’s support act. She seemed to relish the occasion, striding into the crowd during ‘Babygirl’ and playing bass-so-heavy-the-ground-rumbles new tune ‘Bounce’.

She was followed on the Main Stage by The Kills, who kept banter to a minimum during the early part of their set, remaining as quiet as Shifty Shellshock since the release of ‘Butterfly’. However, before ‘Doing It To Death’ Jamie Hince did tell the Polish audience: “Sorry it’s taken us so long to come back, thanks for waiting.” Hince and Mosshart sat together on the lip of the stage for a particularly heartfelt rendition of ‘Monkey 23’. Before closing song ‘No Wow’, Hince announced: “I love you to death! Foo Fighters up next.” Mosshart neglected to mention she’d be joining them.

Open’er Festival continues this week with headline performances from The Weeknd tonight and then The xx and Lorde on Saturday.